Digital printing process and system

Abstract

Embodiments of the invention relate to a method of indirect printing with an aqueous ink. In some embodiments, an intermediate transfer member (ITM) comprising a silicone-based release layer surface is employed. For example, the release layer surface satisfies at least one of the following properties: (i) a receding contact angle of a drop of distilled water deposited on the silicone-based release layer surface is at most 60°; and (ii) a 10-second dynamic contact angle (DCA) of a drop of distilled water deposited on the silicone-based release layer surface is at most 108°. Related apparatus, systems and treatment formulations are disclosed herein.

What is claimed is: 
     
       1. A method of printing comprising:
 a. providing an intermediate transfer member (ITM) comprising a silicone-based release layer surface that is sufficiently hydrophilic to satisfy at least one of the following properties: (i) a receding contact angle of a drop of distilled water deposited on the silicone-based release layer surface is at most 60°; and (ii) a 10-second dynamic contact angle (DCA) of a drop of distilled water deposited on the silicone-based release layer surface is at most 108°; 
 b. providing an aqueous treatment formulation comprising: 
 i. at least 1%, by weight, of at least one water soluble polymer having a solubility in water of at least 5% at 25° C.; and 
 ii. a carrier liquid containing water, said water making up at least 50%, by weight of the aqueous treatment formulation; 
 c. applying the aqueous treatment formulation to the silicone-based release layer surface of the ITM to form thereon a wet treatment layer having a thickness of at most 0.8 μm wherein formation of the wet treatment layer or thinning thereof comprises forcing the aqueous treatment formulation to flow such that a velocity gradient normal to the ITM is established, a magnitude of the velocity gradient being at least 10 6  sec −1 ; 
 d. subjecting the wet treatment layer to a drying process to form a dried treatment film, from the wet treatment layer, on the silicone-based release layer surface 
 e. depositing droplets of an aqueous ink onto the upper surface of the dried treatment film to form an ink image on the silicone-based release layer surface; 
 f. drying the ink image to leave an ink-image residue on the silicone-based release layer surface; and 
 g. transferring the ink-image residue onto a printing substrate by pressured contact between the ITM and the printing substrate. 
 
     
     
       2. The method of  claim 1 , wherein a 60° C. evaporation load of the provided aqueous treatment formulation is at most 8:1. 
     
     
       3. The method of  claim 1 , wherein the drying of the treatment solution is performed sufficiently rapidly so that the thickness of the continuous hydrophilic and cohesive polymer treatment film is at most 150 nm. 
     
     
       4. The method of  claim 1 , wherein a thickness of the dried treatment film is at most 150 nm. 
     
     
       5. The method of  claim 1 , wherein the dried treatment film is sufficiently cohesive such that during transfer of the ink-image residue, the dried treatment film completely separates from the ITM and transfers to the printing substrate with the dried ink image, both in printed and non-printed areas. 
     
     
       6. The method of  claim 1 , wherein the dried treatment film to which the aqueous ink droplets are deposited and a surface of the dried treatment film are characterized by a dimensionless ratio between (i) an average roughness R a  and (ii) a thickness of the dried treatment laver, wherein said dimensionless ratio is at most 0.15. 
     
     
       7. The system of  claim 1  wherein a static surface tension of the aqueous treatment formulation is within a range of 20 and 40 dynes/cm at 25° C. 
     
     
       8. The system of  claim 7  wherein a 60° C. evaporation load of the aqueous treatment formulation is at most 8:1, by weight. 
     
     
       9. The system of  claim 8  wherein a 25° C. dynamic viscosity of the aqueous treatment formulation is at least 10 cP. 
     
     
       10. The system of  claim 7  wherein a 25° C. dynamic viscosity of the aqueous treatment formulation is at least 10 cP. 
     
     
       11. The method of  claim 1  wherein the drying of the treatment solution is performed sufficiently rapidly such that the viscosity of the aqueous treatment formulation increases rapidly enough to inhibit surface-tension-driven beading such that: i. the dried treatment film has a smooth upper surface; and ii. the smooth upper surface of the dried treatment film is characterized by an average roughness Ra of at most 12 nanometers. 
     
     
       12. The method of  claim 11  wherein the magnitude of the velocity gradient is at least 2*10 6  sec −1 . 
     
     
       13. The method of  claim 1  wherein the magnitude of the velocity gradient is at least 2*10 6  sec −1 . 
     
     
       14. A printing system comprising:
 a. an intermediate transfer member (ITM) comprising a silicone-based release layer surface that is sufficiently hydrophilic to satisfy at least one of the following properties: (i) a receding contact angle of a drop of distilled water deposited on the silicone-based release layer surface is at most 60°; and (ii) a 10-second dynamic contact angle (DCA) of a drop of distilled water deposited on the silicone-based release layer surface is at most 108°; 
 b. a quantity of an aqueous treatment formulation comprising: 
 i. at least 1%, by weight, of at least one water soluble polymer having a solubility in water of at least 5% at 25° C.; and 
 ii. a carrier liquid containing water, said water making up at least 50%, by weight of the aqueous treatment formulation; 
 c. a treatment station configured to apply the aqueous treatment formulation to the silicone-based release layer surface of the ITM to form thereon a wet treatment layer having a thickness of at most 0.8 μm, the treatment station is configured such that formation of the wet treatment layer or thinning thereof comprises forcing the aqueous treatment formulation to flow such that a velocity gradient normal to the ITM is established, a magnitude of the velocity gradient being at least 10 6  sec −1 ; 
 d. an image forming station disposed downstream of the treatment station, the image forming station configured to form ink images upon the silicone-based release layer surface of the ITM; and 
 e. an impression station at which the ink images are deposited from the ITM surface to substrate, wherein the system is configured such that: 
 A. the wet treatment layer is subjected to a drying process on the ITM at locations downstream of the treatment station and upstream of the image forming station; 
 B. the drying process is effective to form a dried hydrophilic treatment film from the wet treatment layer; and 
 C. at the image forming station, the ink images formed by depositing droplets of an aqueous ink onto the upper surface of the dried treatment film which is present on the silicone-based release layer surface of the ITM. 
 
     
     
       15. The system of  claim 14  wherein the system is further configured such that the drying of the treatment solution is performed sufficiently rapidly to inhibit surface-tension-driven beading such that:
 i. the dried treatment film has a smooth upper surface; and 
 ii. the smooth upper surface of the dried treatment film is characterized by an average roughness Ra of at most 12 nanometers. 
 
     
     
       16. The system of  claim 15  wherein the treatment station is configured such that the magnitude of the velocity gradient is at least 2*10 6  sec −1 . 
     
     
       17. The system of  claim 14  wherein the treatment station is configured such that the magnitude of the velocity gradient is at least 2*10 6  sec −1 .